1. Field of the Invention
The present invention generally relates to an optical disk device and, more particularly, to an optical disk device reproducing information from an optical disk or recording and reproducing information to/from an optical disk.
2. Description of the Related Art
A write-once optical disk, such as a CD-R (Compact Disk-Recordable), and a DVD-R (Digital Versatile Disk-Recordable), and a rewritable optical disk, such as a CD-RW (CD-Rewritable), a DVD-RW (DVD-Rewritable), a DVD-RAM, and an MO (Magneto Optical disk), are provided with guiding grooves beforehand. In these optical disks, the grooves and lands (parts between the grooves) are determined as tracks. A light beam is projected along these tracks so as to form pits on the tracks; thereby, information is recorded. In this course, a tracking servo used for projecting the light beam along the tracks is controlled such that the center of an optical axis of the light beam coincides with the centerline of the track.
The grooves wobble slightly in a radial direction at a center frequency of 22.05 kHz. Address information (i.e., time information) upon recording, referred to as ATIP (Absolute Time In Pregroove), is multiplexed by an FSK modulation with the maximum excursion of xc2x11 kHz, and is recorded on the grooves as a wobble signal. This wobble signal has a micro amplitude, and thus, does not interfere with the tracking servo. Additionally, the frequency of the wobble signal is out of a control frequency band of the tracking servo; therefore, the center of the optical axis of the light beam traces the centerline of the track on average.
Accordingly, in an optical disk device, an optical detector, which has a light-receiving surface divided into two in the radial direction of the optical disk, receives the light beam reflected on the optical disk. Then, photoelectric conversion signals of this light-receiving surface divided in two are subjected to a differential amplification so that the wobble signal is detected. Then, a spindle motor is revolved, based on a carrier-wave signal at a frequency of 22.05 kHz of the wobble signal so as to revolve the optical disk at a predetermined revolving velocity. Further, the wobble signal is demodulated so as to detect the address information.
On the other hand, upon reproducing, the light beam is projected on the pits formed on the tracks, while being controlled such that the center of the optical axis of the light beam coincides with the centerline of the track. Then, the optical detector, which has the light-receiving surface divided into two in the radial direction of the optical disk, receives the light beam reflected on the optical disk. Then, photoelectric conversion signals of this light-receiving surface divided in two are added so that a recorded signal is detected. This addition offsets the wobble signal component.
FIG. 1 is a block diagram of an example of a signal reproduction circuit of a conventional optical disk device. In FIG. 1, a terminal 10 is supplied with a reproduction RF signal obtained by projecting a light beam from a laser diode of an optical pickup to the optical disk revolved at a predetermined revolving velocity, detecting the reflected light by the optical detector of the optical pickup, and adding the photoelectric conversion signals of the light-receiving surface divided in two. Direct-current components of the reproduction RF signal are removed in a capacitor 11 functioning as a high-pass filter, and the reproduction RF signal is supplied to a noninverting input terminal of a comparator 12. A fixed reference value is supplied from a reference voltage source 13 to an inverting input terminal of the comparator 12. The comparator 12 compares the RF signal with the reference value so as to make the RF signal binary. This binary signal is supplied to a PLL (Phase Locked Loop)/detector 16.
The PLL/detector 16 generates, in a PLL thereof, a clock PCLK synchronized with the supplied binary signal, and outputs the clock PCLK via a terminal 18. Also, the PLL/detector 16 detects, in a detector thereof, the presence of a reproduction pulse by a detecting window determined based on the clock PCLK so as to discriminately reproduce a signal REFM and output the signal REFM via a terminal 19.
FIG. 2 is a block diagram of another example of a signal reproduction circuit of a conventional optical disk device. In FIG. 2, the terminal 10 is supplied with the reproduction RF signal obtained by projecting the light beam from the laser diode of the optical pickup to the optical disk revolved at a predetermined revolving velocity, detecting the reflected light by the optical detector of the optical pickup, and adding the photoelectric conversion signals of the light-receiving surface divided in two. The direct-current components of the reproduction RF signal are removed in the capacitor 11 functioning as a high-pass filter, and the reproduction RF signal is supplied to the noninverting input terminal of the comparator 12. A threshold value corresponding to a midpoint potential of the RF signal is supplied from a low-pass filter/amplifier (LPF/AMP) 14 to the inverting input terminal of the comparator 12. The comparator 12 compares the RF signal with the threshold value so as to make the RF signal binary. This binary signal is supplied to the low-pass filter/amplifier 14 and the PLL/detector 16.
The low-pass filter/amplifier 14 integrates the binary signal, and thereafter, amplifies the integrated value with a predetermined gain so as to generate the threshold value corresponding to the midpoint potential of the RF signal. Then, the low-pass filter/amplifier 14 supplies the threshold value to the comparator 12. The comparator 12 and the low-pass filter/amplifier 14 form an asymmetry correction circuit, which determines the threshold value such that high-level periods and low-level periods of the binary signal become equal in total. A response characteristic of this asymmetry correction circuit is optimized by adjusting a time constant and a gain of the low-pass filter/amplifier 14, an amplitude of the RF signal, and an output voltage of the comparator 12.
The PLL/detector 16 generates, in the PLL thereof, the clock PCLK synchronized with the supplied binary signal, and outputs the clock PCLK via the terminal 18. Also, the PLL/detector 16 detects, in the detector thereof, the presence of a reproduction pulse by the detecting window determined based on the clock PCLK so as to discriminately reproduce the signal REFM and output the signal REFM via the terminal 19.
By the way, the light beam projected from the laser diode has different powers as a write power and a read power; therefore, the optical axis may possibly be displaced. In FIG. 3, the light beam as the read power is projected from the laser diode as indicated by solid lines, and the light beam as the write power is projected from the laser diode as indicated by broken lines. Therein, the optical axis is displaced by an angle xcex8. When the optical axis is displaced in a widthwise direction of the groove, pits recorded by the write power are displaced from the centerline of the track, as shown in FIG. 4, because, even upon recording, a tracking error signal is generated at a read-power timing. Additionally, there are other cases in which, for example, the pits are displaced from the centerline of the track, due to remaining heat of the adjacent track after the completion of a recording.
When the groove is used as a track, and the light beam is projected, with the optical axis thereof being displaced from the centerline of the groove, the light beam comes near to the centerline of the groove or goes far away from the centerline of the groove, depending on a wobbling cycle, because the groove wobbles as described above. When the light beam is projected near the centerline of the groove, the recording becomes normal. However, when the light beam is projected far away from the centerline of the groove, the recording becomes insufficient by being influenced by an end portion of the groove. Thus, the pits are influenced by the wobbling cycle.
Upon reproducing, a tracking error signal is generated from the pits and the groove such that the light beam substantially passes through the vicinity of the center of the pit. However, an amplitude fluctuation or a waveform distortion of the wobbling frequency occurs in the reproduction RF signal, due to the influence made upon the recording. This influence becomes greater as the cycle of the recorded signal becomes shorter; in a CD, a signal of 3T (a reference time period T is approximately 230 nsec, i.e., one cycle of a frequency of 4.32 MHz at a normal speed, i.e., a single-fold speed) is influenced the most. FIG. 5 shows the reproduction RF signal in this case.
If an envelope of the RF signal supplied to the comparator 12 shown in FIG. 1 fluctuates up and down due to the wobble signal component, a reproduction jitter occurs, when the comparator 12 makes the RF signal binary; this causes a problem that the binary signal output by the comparator 12 includes the wobble signal component.
In the asymmetry correction circuit of the conventional signal reproduction circuit shown in FIG. 2, since the RF signal is made binary exactly in accordance with a scratch, a spot, and unsteady reflectance on the surface of the optical disk, a response time constant is set sufficiently high so as not to interfere with an EFM frequency component. However, since a frequency band of the wobble signal component partially overlaps a frequency band of the EFM frequency component, the wobble signal component is not extracted in the low-pass filter/amplifier 14. Therefore, the asymmetry correction circuit responds to the above-mentioned wobble signal component included in the RF signal supplied to the comparator 12, and the threshold value supplied to the comparator 12 fluctuates in response to the wobble signal component. The reproduction jitter deteriorates due to this fluctuation of the threshold value.
It is a general object of the present invention to provide an improved and useful optical disk device in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an optical disk device which can reduce a reproduction jitter resulting from a wobble signal component superimposed on a reproduction RF signal.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention an optical disk device reproducing an RF signal by projecting a light beam on an optical disk and detecting a reflected light therefrom, information being recorded on the optical disk by forming a pit on a track determined according to a groove having a wobble signal recorded thereon, the device comprising:
a comparator comparing the RF signal with a threshold value so as to output a binary signal;
a first band-pass filter filtering a frequency band of the wobble signal from the binary signal so as to extract a wobble signal component; and
a first adder obtaining a value by adding the wobble signal component to a fixed reference value so as to supply the value to the comparator as the threshold value.
According to the present invention, the wobble signal component superimposed on the RF signal and the wobble signal component of the threshold value are offset so as to reduce the generation of a reproduction jitter upon making the RF signal binary.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention an optical disk device reproducing an RF signal by projecting a light beam on an optical disk and detecting a reflected light therefrom, information being recorded on the optical disk by forming a pit on a track determined according to a groove having a wobble signal recorded thereon, the device comprising:
a comparator comparing the RF signal with a threshold value so as to output a binary signal;
a second band-pass filter filtering a frequency band of the wobble signal from the RF signal so as to extract a wobble signal component; and
a second adder obtaining a value by adding the wobble signal component to a fixed reference value so as to supply the value to the comparator as the threshold value.
According to the present invention, the wobble signal component superimposed on the RF signal and the wobble signal component of the threshold value are offset so as to reduce the generation of a reproduction jitter upon making the RF signal binary.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention an optical disk device reproducing an RF signal by projecting a light beam on an optical disk and detecting a reflected light therefrom, information being recorded on the optical disk by forming a pit on a track determined according to a groove having a wobble signal recorded thereon, the device comprising:
a comparator comparing the RF signal with a threshold value so as to output a binary signal;
a first band-pass filter filtering a frequency band of the wobble signal from the binary signal so as to extract a wobble signal component;
an integrator integrating the binary signal so as to generate an integrated reference value; and
a third adder obtaining a value by adding the wobble signal component to the integrated reference value so as to supply the value to the comparator as the threshold value.
According to the present invention, the wobble signal component superimposed on the RF signal and the wobble signal component of the threshold value are offset so as to reduce the generation of a reproduction jitter upon making the RF signal binary.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention an optical disk device reproducing an RF signal by projecting a light beam on an optical disk and detecting a reflected light therefrom, information being recorded on the optical disk by forming a pit on a track determined according to a groove having a wobble signal recorded thereon, the device comprising:
a comparator comparing the RF signal with a threshold value so as to output a binary signal;
a second band-pass filter filtering a frequency band of the wobble signal from the RF signal so as to extract a wobble signal component;
an integrator integrating the binary signal so as to generate an integrated reference value; and
a fourth adder obtaining a value by adding the wobble signal component to the integrated reference value so as to supply the value to the comparator as the threshold value.
According to the present invention, the wobble signal component superimposed on the RF signal and the wobble signal component of the threshold value are offset so as to reduce the generation of a reproduction jitter upon making the RF signal binary.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention an optical disk device reproducing an RF signal by projecting a light beam on an optical disk and detecting a reflected light therefrom, information being recorded on the optical disk by forming a pit on a track determined according to a groove having a wobble signal recorded thereon, the device comprising:
a comparator comparing the RF signal with a threshold value so as to output a binary signal;
a first band-pass filter filtering a frequency band of the wobble signal from the binary signal so as to extract a wobble signal component;
an integrator integrating the binary signal so as to generate an integrated reference value and supply the integrated reference value to the comparator as the threshold value; and
a subtracter subtracting the wobble signal component from the RF signal so as to supply the RF signal to the comparator.
According to the present invention, the wobble signal component superimposed on the RF signal is removed so as to reduce the generation of a reproduction jitter upon making the RF signal binary.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.